Field of the Disclosure
The present disclosure relates to an image forming apparatus that uses electrophotography, such as a copier or printer or the like.
Description of the Related Art
There conventionally have been known color image forming apparatuses that use electrophotography, where toner images are sequentially transferred from image forming units of each color onto an intermediate transfer medium, following which the toner images are transferred to a transfer medium en bloc. In such image forming apparatuses, each image forming unit for each color has a drum-shaped photosensitive member (hereinafter referred to as “photosensitive drum”) serving as an image bearing member. Toner images formed on the photosensitive drums of the image forming units are transferred by primary transfer onto the intermediate transfer member such as an intermediate transfer belt or the like, by application of voltage from a primary transfer power source to a primary transfer member provided facing the photosensitive drums, with the intermediate transfer member interposed therebetween. The toner images of these colors that have been transferred from the image forming units of each color onto the intermediate transfer member by primary transfer are then transferred en bloc by secondary transfer from the intermediate transfer member onto a transfer medium such as paper, overhead projector (OHP) sheet, or the like, by application of voltage from a secondary transfer power source to a secondary transfer member at a secondary transfer portion. The toner images of each of the colors transferred onto the transfer medium are then fixed onto the transfer medium by a fixing unit.
Japanese Patent Laid-Open No. 2012-098709 discloses a configuration where an intermediate transfer belt having electrical conductivity is used as the intermediate transfer member, and primary transfer of toner images from multiple photosensitive drums to the intermediate transfer belt is performed by electric current supplied from an electric current supply member flowing in the circumferential direction, along the length, of the intermediate transfer belt. However, there is concern that the configuration in Japanese Patent Laid-Open No. 2012-098709 may have difficulty in securing good primary transferability in a case where electrical resistance of the intermediate transfer belt changes. In a configuration where electric current from the electric current supply member flows in the circumferential direction of the intermediate transfer belt, the distance over which electric current for performing primary transfer flows over the intermediate transfer belt is long. In this case, the voltage at a primary transfer portion where a photosensitive drum and the intermediate transfer belt come into contact (hereinafter referred to as primary transfer voltage) drops by an amount corresponding to the current that has flowed in the circumferential direction of the intermediate transfer belt, so the primary transfer voltage is readily affected by change in the electrical resistance of the intermediate transfer belt.
For example, an intermediate transfer belt made up of multiple layers, of which a layer having ionic conductivity is the thickest in the thickness direction of the intermediate transfer belt, tends to exhibit change in electrical resistance due to the ambient environment. More specifically, in a high-temperature high-humidity environment, the electrical resistance of the intermediate transfer belt tends to become low, while in a low-temperature low-humidity environment, the electrical resistance of the intermediate transfer belt tends to become high. Considering a case of applying a voltage to a current supply member so that the primary transfer voltage is a suitable voltage for performing primary transfer under a standard environment, using such an intermediate transfer belt, the amount of drop of primary transfer voltage in a low-temperature low-humidity environment is greater than the amount of drop of primary transfer voltage in a standard environment, so there is a possibility that the primary transfer voltage necessary for performing the primary transfer of a toner image in a photosensitive drum onto the intermediate transfer belt may be insufficient, which may result in image defects. On the other hand, the amount of drop of primary transfer voltage in a high-temperature high-humidity environment is smaller than the amount of drop of primary transfer voltage in a standard environment, so there is a possibility that primary transfer voltage necessary for performing primary transfer of a toner image in a photosensitive drum onto the intermediate transfer belt may be excessive, which may result in image defects.